Emerson Rosemount 848T, Reference Manual, Page: 127 / 136

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Reference Manual
00809-0100-4697, Rev GA
Appendix D: Function Blocks
November 2014
Function Blocks
Alarm detection
A block alarm will be generated whenever the BLOCK_ERR has an error bit set. The type of block
errors for the ISEL block are defined above.
Process Alarm detection is based on the OUT value. The alarm limits of the following standard
alarms can be configured.

High (HI_LIM)

High high (HI_HI_LIM)

Lo (LO_LIM)

Lo low (LO_LO_LIM)
In order to avoid alarm chattering when the variable is oscillating around the alarm limit, an
alarm hysteresis in percent of the PV span can be set using the ALARM_HYS parameter. The
priority of each alarm is set in the following parameters:

HI_PRI

HI_HI_PRI

LO_PRI

LO_LO_PRI
Table D-8. Alarm Priority Levels
Block execution
The ISEL function block reads the values and status of up to eight inputs. To specify which of the
six available methods (algorithms) is used to select the output, configure the selector type
parameter (SELECT_TYPE) as follows:

Max selects the maximum value of the inputs.

Min selects the minimum value of the inputs.

Avg calculates the average value of the inputs.

Mid calculates the update for eight sensors.

1st Good selects the first available good input.
If the DISABLE_N is active, the associated input is not used in the selection algorithm.
If an input is not connected, it is also not used in the algorithm.
Number Description
0 The priority of an alarm condition changes to 0 after the condition that caused the alarm is
corrected.
1 An alarm condition with a priority of 1 is recognized by the system, but is not reported to
the operator.
2 An alarm condition with a priority of 2 is reported to the operator, but does not require
operator attention (such as diagnostics and system alerts).
3-7 Alarm conditions of priority 3 to 7 are advisory alarms of increasing priority.
8-15 Alarm conditions of priority 8 to 15 are critical alarms of increasing priority.